1. Field of the Invention
The present invention relates to a honeycomb structure.
2. Description of the Related Art
Conventionally, as a honeycomb catalyst used for purifying exhaust gas of vehicles, platinum is carried on a layer made of activated alumina and the like having a larger specific surface area, the layer being formed on a surface of a honeycomb structure made of cordierite. Further, as a honeycomb catalyst for converting exhaust gas of diesel engines, NOx storage agent is further carried so as to treat NOx in an oxygen-enriched atmosphere.
The NOx storage agent, however, is more likely to stably store SOx rather than NOx. Because of this feature, SOx poisoning in which the NOx storage agent stores SOx occurs and, disadvantageously NOx cannot adequately stored.
To solve the problem, according to Japanese Patent Laid-Open Publication No. 6-58138, a sulfur capturing device is provided in an exhaust gas passage on an upstream side with respect to the place where NOx storage agent is provided. The sulfur capturing device includes a sulfur absorbing agent and a casing enclosing the sulfur absorbing agent. As the sulfur absorbing agent, a noble metal such as platinum in addition to any one metal selected from a group consisting of an alkali metal such as potassium, sodium, lithium, and cesium, an alkali-earth metal such as barium an calcium, and a rare earth metal such as lanthanum and yttrium are carried on alumina carrier. Further, a honeycomb structure as described in WO 05/063653A is disclosed.
The contents of Japanese Patent Laid-open Publication No. 6-58138 and WO 05/063653A are incorporated by reference in their entirety.